In optical communication systems, a light beam is modulated in accordance with information to be conveyed and transmitted along dielectric optical waveguides to a receiver. Typically, transmission of broadband signal content, such as analog multichannel video, requires the use of narrow line width light sources in conjunction with low loss, single mode optical fibers (SMF). By way of example, a typical transmitter for cable TV (CATV) operates at a wavelength of 1550 nm and includes the use of a narrow line width, continuous wave (CW) distribution feedback (DFB) laser and an external modulator.
Long distance transmission in optical fiber typically requires high fiber launch power, for instance, to achieve required signal to noise ratio. This is especially true for analog transmission systems for hybrid fiber-coax (HFC) networks and passive optical networks (PON) in which high fiber launch power enables the high signal to noise ratio requirement to be achieved and permits higher split numbers along the transmission line.
The success of optical amplifiers, such as erbium doped fiber amplifiers (EDFA) and semiconductor optical amplifiers (SOA), has essentially eliminated high launch power as a problem in most optical fiber communication applications. This is because these amplifiers permit efficient signal amplification of optical carriers around 1550 nm up to saturated output powers exceeding about 23 dBm (200 mW). This enables longer reach fiber links and the ability to optically split the signal to serve multiple users.
However, despite the use of such optical amplifiers, fiber nonlinearities limit maximum launch power into optical fiber. In particular, for a single wavelength system, stimulated Brillouin scattering (SBS) puts a limit to maximum launch power in many communication applications before the impact of other fiber nonlinearities becomes relevant. With respect to SBS impact, SBS typically only occurs when a narrow line width optical beam is launched into an optical fiber above a threshold power level. Thus, as long as the power within the SBS line width does not exceed the SBS threshold power level, SBS should remain adequately suppressed. Unfortunately, the SBS threshold power level, for instance, for standard SMF is typically in a range of only about 6-7 dBm (4-5 mW). Thus, raising the SBS threshold above 6-7 dBm (4-5 mW) so that launch power can be increased is desirable in many applications.
Brillouin scattering is an interaction of light photons with acoustic or vibrational quanta (phonons). The interaction consists of an inelastic scattering process in which a phonon is either created (Stokes process) or annihilated (anti-Stokes process). The energy of the scattered light is slightly changed, that is decreased for a Stokes process and increased for an anti-Stokes process. This shift, known as the Brillouin shift, is equal to the energy of the interaction. For intense laser light traveling in an optical fiber of very small core diameter, the variations in the electric field of the beam itself may produce acoustic vibrations in the medium via electrostriction. The beam may undergo Brillouin scattering from these vibrations, usually in opposite direction to the incoming beam.
SBS affects optical transmission systems within an optical channel and normally will not cause crosstalk between multiple optical channels because of its narrow gain spectral width. However, because of its narrow bandwidth nature, SBS is particularly detrimental to optical transmission systems having modulation schemes which generate narrow optical spectrum where most of the optical power centers in a small frequency range near optical carrier. As an example, the modulation schemes of CATV/HFC systems are typically amplitude modulation with vestigial sideband (AM-VSB) whose root mean square (RMS) modulation index is about 20% to 30% without laser clipping. Therefore, most of the energy centers within a small bandwidth around the optical carrier. Thus, SBS impacts CATV/HFC systems on carrier to noise ratios (CNR) and distortions, especially second order distortion, CSO.
In CATV/HFC systems, SBS affects the externally modulated analog systems much more than directly modulated analog systems. A first reason for this is that an external modulator exhibits almost zero modulator chirp, and thus, the power is densely centered closely around optical carrier. A second reason is that external modulators are used for longer reach because of its low chirp, and long reach requires more launch power. In directly modulated analog systems, on the other hand, a broadened optical spectrum due to the relative larger laser chirp, together with fiber dispersion, restricts link length due to performance degradation. For this reason, the directly modulated analog systems are usually used for shorter reach with a relatively lower launch power. Hence, the analog transmission systems of directly modulated lasers are less susceptible to SBS.
In general, SBS impact can be reduced in an externally modulated analog system if the optical signal's spectrum can be broadened since the energy per bandwidth is lowered. The most effective and widely used techniques for combating SBS include the use of an optical phase modulator or dithering the laser or the combination of both, in the case of external modulators.
Accordingly, the transmission quality of optical signals having relatively high intensity and narrow line width can be improved by reducing the effects of SBS which allows increase of optical signal power level and increase of propagation distance between communication links without generating additional system degradation.